understanding memory usage in Android

i am trying to understand memory usage in Android.

when I run Norton Mobile Utilities, I see: Resident Memory, Shared Memory and Effective Memory - can someone explain the differen开发者_如何学运维ce between these?

Also, when I dump an HPROF file from eclipse, it says I'm using like 2.6MB of memory, but when I bring up a utility like Norton, it says I am using 31MB of resident memory, 18MB of shared memory and 13MB of effective memory -- say what??